This invention relates to a gear having double tooth rows the tooth positions of which deviate from each other in the pitch circle direction.
Gears engaged with each other by and large bring about vibrations and noises when rotating to transmit a rotary motion. The vibrations and noises produced during the rotation of the engaged gears are due to backlash (gap between meshed teeth) which is intentionally provided between the meshed teeth of the engaged gears in order to smoothly rotate the gears to transmit the rotary motion and deviation in circular pitch attributable to an error in measurement in manufacturing the gear. Thus, the aforementioned backlash (clearance) is necessary for making up for the deviation in circular pitch. Therefore, the conventional gears cannot essentially be free from such disadvantages.
Furthermore, the gears inevitably turn out to be worn during service to make the backlash larger. As a result, the vibrations and noises produced in rotating the engaged gears become conspicuous. Though a gear system with a structure capable of positively lessening the backlash might be able to be accomplished, it would be complicated in structure and difficult to manufacture.
Helical gears can decrease vibrations and noises produced during rotation and transmit a rotary motion smoothly, but entail problems such as difficulty encountered in the manufacture thereof and production of undesirable thrust stress. Though double helical gears produce little thrust stress, the difficulty in the manufacture of the helical gear remains the same. Beyond these drawbacks suffered by the gears of this type, vibrations and noises produced inherently in rotating the engaged gears can not be prevented.
On the other hand, a spur gear having a row of teeth extending in parallel to the axis thereof per se does not withstand the thrust load exerted on the gear teeth in the axial direction.
Under the circumstances noted above, a need has been felt for a high-performance spur gear capable of efficiently transmitting a rotary motion without bringing about vibrations and noises and being especially useful in precision instruments and machines.